1. Field of the Invention
The present invention relates to a high-frequency wiring board for achieving connection and mounting of a high-frequency integrated circuit such as an IC and LSI or a high-frequency circuit device used in a millimeter-wave high-frequency band, and more particularly to a high-frequency wiring board having a signal-transmission through conductor that allows improvement in high-frequency signal transmission characteristics.
2. Description of the Related Art
FIGS. 6 and 7 are a plan view and a sectional view, respectively, showing one example of a conventional high-frequency wiring board for transmitting a millimeter-wave-band high-frequency signal.
In FIGS. 6 and 7, reference numeral 21 denotes a high-frequency wiring board and reference numeral 22 denotes a dielectric substrate. The dielectric substrate 22 has a first line conductor 23 and a second line conductor 24 respectively formed on the top and bottom surfaces thereof. The first and second line conductors 23 and 24 are electrically connected to each other, at their one ends, by a through conductor 25. The dielectric substrate 22 also has a first coplanar ground conductor 26 and a second coplanar ground conductor 27 respectively formed on the top and bottom surfaces thereof. The first and second coplanar ground conductors 26 and 27 are electrically connected to each other by a plurality of grounding through conductors 28.
The high-frequency wiring board 21 has the following drawback. Due to a stray capacitance present in the joint between the first and second line conductors 23, 24 and the through conductor 25, there occurs characteristic impedance mismatch, resulting in an increase in reflection loss of high-frequency signals. This causes degradation of the transmission characteristics.
To overcome such a problem, for example, a technique for achieving characteristic impedance matching in the joint between the line conductor and the through conductor has been proposed to date (refer to Japanese Unexamined Patent Publication JP-A 2000-100993 (2000)). According to this technique, by increasing the interval between the line conductor and the nearby coplanar ground conductor partly in the vicinity of the joint between the line conductor and the through conductor, the stray capacitance can be reduced, whereby making it possible to achieve characteristic impedance matching in the joint between the line conductor and the through conductor. As a result, the high-frequency signal transmission characteristics can be enhanced.
However, the conventional high-frequency wiring board described just above also has the following drawback. In the case of increasing the interval between the line conductor and the nearby coplanar ground conductor in the vicinity of the joint between the line conductor and the through conductor, the coplanar ground conductor located immediately below each of the line conductors is absent in the vicinity of the through conductor. In the millimeter-wave frequency band, the high-frequency signal has a short wavelength, and thus characteristic impedance mismatch occurs in the vicinity of the joint between the line conductor and the through conductor. As a result, the higher the frequency, the poorer the transmission characteristics can be.